Discussions of Related Art
Dental porcelain suffers from the phenomenon of being changed to yellowish color when fired to bond (baked) to a metallic frame containing particular metal components, above all, silver (Ag), or when fired in an environment in which such metal components co-exist. This phenomenon, referred to hereinafter as yellow coloration, is fatal to the dental porcelain, the color tone of which needs to be conditioned carefully after firing. Although various measures have so far been taken to eliminate this problem. None of these measures is fully satisfactory. Some of the defects inherent in these conventional measures are discussed hereinbelow.
Yellow coloration by silver is diminished to some extent by a method comprising placing carbon honeycomb and a carbon plate side by side with dental porcelain in a furnace. However, not only is it possible with this method to prevent yellow coloration completely, but the porcelain tends to be faded in color due to oxides under the reducing action caused by CO gas. In addition, air bubbles tend to be produced in the porcelain.
There has also been proposed a method consisting in previously coating a metallic conditioner on a porcelain-baking surface of a metallic frame or the like. However, with this method, the bonding strength of the porcelain to the frame tends to be lowered. Besides, silver is volatilized from the inside of a non-processed metallic frame or from the interior of the furnace contaminated with silver, so that yellow coloration of the porcelain can not be prevented completely. Besides, if the porcelain is post-brazed to an artificial crown formed of an Au--Pd--Ag alloy or a Pt--Au based alloy containing about 10% of silver, yellow coloration of the porcelain is unavoidably produced due to silver contained in these alloys.
If a metal frame is produced using semi-precious grade alloys free from silver, the porcelain fired to bond to the metal frame is hardly subjected to yellow coloration. However, if a silver-containing artificial cast crown is post-brazed to the metallic frame, yellow coloration is unavoidably produced at a boundary region. However, if the artificial crown is post-brazed to the metallic frame free of silver, the bonding strength is so low that fracture tends to be produced within the oral cavity. Thus, one is frequently obliged to prefer the method of firing the porcelain to bond to the silver-containing alloy frame to endure the phenomenon of yellow coloration.
Recently, a porcelain material containing nitrate compounds has been proposed, as in JP Patent KOKAI Publication No. 63-310805(1988). However, the resulting porcelain tends to suffer from opacification after firing. On the other hand, yellow coloration cannot be prevented satisfactorily if the porcelain is repeatedly subjected to firing. That is, although yellow coloration can be effectively prevented as long as the first firing is concerned, the nitrate compounds are dissipated by the initial firing and become hardly effective during the subsequent firing process so that yellow coloration is ultimately produced.